A vehicle equipped with a hybrid propulsion system, i.e. a vehicle integrating an electrical motor and an internal combustion engine, typically operates by selectively employing the electrical motor and the internal combustion engine in the driveline to propel the vehicle. One such hybrid propulsion system, referred to as vehicle stop/start system, comprises a system which turns off the internal combustion engine during vehicle idle/stop, and enables early fuel cutoff to the engine during decelerations. When an operator subsequently tips into the throttle to accelerate the vehicle, a control system causes the electric motor to crank the internal combustion engine if engine speed is below a certain threshold, using electrical energy stored in a high-voltage battery. This operation is intended to provide improved fuel economy in stop-and-go driving. The high-voltage battery requires recharging to replenish the stored electrical energy consumed by the electric motor.
One exemplary hybrid system designed to achieve vehicle stop/start operation comprises a belt alternator starter (BAS) system. The hybrid system combines sophisticated engine controls with a precision electric motor/generator. Regenerative braking and efficient electrical charging are included in such a system. A single, 36-volt battery provides electrical power, augmenting the existing 12-volt electrical system. All vehicle accessories and passenger comfort systems, such as air conditioning, function even during the periods when the engine is automatically stopped. The BAS system may be employed on four and six-cylinder engines with minimal effect on engine and transmission architectures.
It is well understood that maintaining state of charge (‘SOC’) of the electrical energy storage system, e.g., the high-voltage battery, is important. Over-depletion of battery SOC typically decreases battery life, whereas overcharging of the battery unnecessarily expends fuel, thus decreasing effective fuel economy of the vehicle.
Prior efforts to control battery charging include zero-current charging, or ‘trickle charging’, wherein there is continual charging of the battery at near zero current. At light motor-generator unit (MGU) loads, the battery charging unit is not operating at optimal efficiency, thus needlessly expending energy. Furthermore, opportunities to more completely charge the battery are missed at higher engine efficiencies, due to current-limiting characteristics of the zero-current system.
It is important to optimize battery charging, to maximize battery life and maximize vehicle fuel economy in a hybrid electric vehicle. The disclosed invention provides optimized battery charging that addresses the concerns mentioned above.